The Types Of Modem

Published Date: 02 Nov 2017

Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps), or bytes per second (B/s). Modems can alternatively be classified by their symbol rate, For example, the ITU V.21 standard used audio frequency shift keying with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which was able to transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.

ORIGIN AND HISTORY:-

Modems grew out of the need to connect teleprinters over ordinary phone lines instead of more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. In 1942, IBM adapted this technology to their unit record equipment and was able to transmit punched cards at 25 bits/second

Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.

In the summer of 1960, the name Data-Phone was introduced to replace the earlier term digital subset. The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960 in 1962, the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels running on their own set of two wires each.

The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, the ASR33, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.

COMPONENTS:-

As technology advances, functions that are provided by multiple chips can be integrated onto one chip. Higher levels of integration have benefited DSL just as they benefited other computer hardware. A DSL modem requires the following for its operation; exactly what is on the circuit card and how it is arranged can change as technology improves:

Power supply: Transformer and capacitor

Data connection and power circuitry (for example, USB, Ethernet, PCI)

DSL digital data pump

DSL analog chip and line driver

Micro controller

Filter

Transistor

WORKING:-

As we know that the computers can’t understand our language. Computers understand the language of 0’s and 1’s i.e. digital form. The modem will modulate the signal into a sine wave. So, this modulated digital signal can easily run over the telephone lines. Then the signal will reach the IP hub. After this the demodulation of the signal will take place into digital form. Then you will be connected to the internet. All these processes are performed by modem at extremely high speeds. The speed of the modem depends upon the number of the available access lines and the technology of the modem.

The features of modem are discussed below:

1. The speed of the modem is measured in bps means bits per second. Data transfer speed can be increased by using the technique of data compression.

2. If we are using a modem having auto answering facility. Then our modem will be able to attend calls even in our absence.

3. Modems work basically in two modes. One is voice mode and other is data mode. In voice mode modem acts like a simple telephone. But in data mode modem acts as a Simple modem. These types of modems have a switch which is helpful in changing the mode i.e. from voice mode to data mode and from data mode to voice mode. For voice communication, loudspeaker and a microphone is implemented in the modem.

4. Some modems have the ability to compress data. These modems compress data before sending to improve the data transfer rates. But there must be a similar technology modem to decompress the data at the receiver end.

TYPES OF MODEM:-

There are basically three types of modems. All the three types are discussed in detail below:

a)External Modems

b) Internal Modems

c) PCMCIA Modem

External Modems: A modem that resides in a self-contained box outside the computer system. Contrast with an internal modem, which resides on a printed circuit board inserted into the computer.

External modems tend to be slightly more expensive than internal modems. Many experts consider them superior because they contain lights that indicate how the modem is functioning. In addition, they can easily be moved from one computer to another. However, they do use up one COM port.

modem

External Modem

Internal Modems: Internal modems are basically integrated on a chip. These are put up into the PCI slots of the computer. There is no need of any external power supply for internal modems. These modems use the power supply of the PC. Their installation in PC is quite very simple.

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PCMCIA Modem:

Short for Personal Computer Memory Card International Association, and pronounced as separate letters, PCMCIA is an organization consisting of some 500 companies that has developed a standard for small, credit card-sized devices, called PC Cards. Originally designed for adding memory toportable computers, the PCMCIA standard has been expanded several times and is now suitable for many types of devices. There are in fact three types of PCMCIA cards. All three have the same rectangular size (85.6 by 54 millimeters), but different widths

Type I cards can be up to 3.3 mm thick, and are used primarily for adding additional ROM or RAM to a computer.

Type II cards can be up to 5.5 mm thick. These cards are often used formodem and fax modem cards.

Type III cards can be up to 10.5 mm thick, which is sufficiently large for portable disk drives.

As with the cards, PCMCIA slots also come in three sizes:

A Type I slot can hold one Type I card

A Type II slot can hold one Type II card or one Type I card

A Type III slot can hold one Type III card or any combination of two Type I or II cards.

In general, you can exchange PC Cards on the fly, without rebooting your computer. For example, you can slip in a fax modem card when you want to send a fax and then, when you're done, replace the fax modem card with a memory card.

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PCMCIA Modem

APPLICATION:-

Advantages and Disadvantages:-

Dial-up Modems

Advantages

Dial-up service requires an inexpensive internal or external modem. Setting up service is quick and easy.

The biggest benefit of using dial-up Internet service is that almost everyone in the U.S. has a telephone line that can be used to connect to the Internet.

In many rural areas, dial-up service is the only service available.

Disadvantages

One disadvantage of dial-up service is using a telephone line to access the Internet.

In some cases, users cannot make outgoing calls or receive incoming calls while online.

Another disadvantage is slow transmission rates compared with broadband. Website pages load slowly, and downloading files is time-consuming.

Cable Modems

Advantages

Setting up cable modem service is simple with an installation kit provided by ISPS

Service is found in most major cities, and it is a high-speed connection.

Once service is installed, it is always on and ready to use, in contrast to dial-up.

Cable modem Internet service does not require a phone line.

Disadvantages

A disadvantage of cable Internet service is that it costs more than DSL or a dial-up connection.

It is available only from specific cable TV networks in your area. Connection speed is determined by the quantity of people in your neighborhood using cable equipment simultaneously as you.

DSL Modem

Advantages

Even though DSL modem service requires a phone line, subscribers can still use phone services while online.

Connections speeds are better than those of dial-up.

DSL services are now more widely available than cable Internet services.

The connection speed will not be diminished by large numbers of other users, as cable modem service is. High-speed DSL is a dedicated line with easy setup.

Disadvantages

A disadvantage is higher costs than dial-up.

DSL modem service is still not available in many remote and rural areas.

Connection speeds will vary depending on the distance between the ISP center and your phone line

In some areas, DSL service is available only through the local phone company.

PRESENT STATE AND FUTURE PROSPECTS:-

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In 2005 there are 30% users of modem and which increases upto 60% till 2012.At present in 2013 it increases 10%.

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Total Broadband users (By Country):

In united states there are 90% users of modem(Broadband) which is maximum in world and minimum in iceland